Inductive decoupling of a rf coil array

ABSTRACT

An apparatus for imaging includes: a radio frequency (RF) coil array having a first RF coil and at least one additional RF coil, where the RF coil array is adapted to generate an image signal; a preamplifier having an input impedance, where the preamplifier is adapted to receive the image signal from the first RF coil; and a transformer to couple the first RF coil to the preamplifier, where impedance of the transformer is adapted to match the input impedance of the preamplifier.

FIELD OF THE INVENTION

This invention generally relates to radio frequency (RF) coil arraysused in magnetic resonance imaging (MRI). Specifically, the inventionrelates to decoupling the coils in a RF coil array for MRI.

BACKGROUND

The elimination of inductive coupling is an important step for the useof RF coils arrays for MRI, such as in parallel imaging for MRI. Forexample, in nuclear magnetic resonance (NMR) imaging, if some effectivemutual inductance remains among the coils in the coil array, the NMRsignal obtained from one coil may disturb the flux in another coil,which may make it difficult to match and tune each circuit with a coilto the input impedance of the respective preamplifier circuit.

A common method to isolate the coils in the coil arrays avoids the buildup of significant currents from the NMR signal among each of the coilsin the coil array, in such way that the effects from the mutualinductances may be neglected. This may be conventionally achieved byconnecting each coil to a circuit that should behave as an open circuitfrom its input port. A common method employed for this circuitimplementation usually involves the use of a network transformer (or animpedance transformer) and preamplifiers with very low input impedance(typically <2Ω) and decoupling networks with lumped elements. However,the construction of low noise figure preamplifiers with extremely lowinput impedance is not easy to accomplish. In addition, the use ofpreamplifiers with low input impedance imposes technical restrictions oncoil design, especially when considering geometries that requireoverlapping loops where there is a significant amount of mutualinductance between the RF coils.

In addition, it is known that the coupling of coils to the matchingnetwork is usually a hard task to accomplish and is highly dependent oneffects caused by sample loading. One such effect is produced by voltagedifferences in different parts of the coil that generate anelectrostatic field around the coil. This electrostatic field may coupleto the sample, causing dielectric losses and consequently a reduction ofthe received signal. Another undesirable effect is caused by standingwaves that may be present in the cables that connect the coils to thepreamplifier, which may feed back to the pickup coil or may alsorepresent a radiation loss for the NMR signal. Conventionally, thesedisturbances in the signal-to-noise ratio may be improved by reducingthe dielectric losses and also by reducing the currents in the groundloops that may provide resistive and radiation losses.

Another technique employed to decouple RF coils in an array attempts tocancel the flux between any two coils in the array. However, cancelingflux is not an efficient method for isolating non-adjacent neighboringcoils. Further, canceling flux tends to degrade the NMR signal byinevitable insertion of losses on coils and, moreover, may not work forarbitrary coil geometries.

SUMMARY

One embodiment of the invention includes an apparatus for imagingincluding: a RF coil array having a first RF coil and at least oneadditional RF coil, the RF coil array adapted to generate an imagesignal; a preamplifier having an input impedance, the preamplifieradapted to receive the image signal from the first RF coil; and a firsttransformer to couple the first RF coil to the preamplifier, impedanceof the first transformer to match the input impedance of thepreamplifier.

One embodiment of the invention includes a method for imaging,including: obtaining an image with a RF coil array including a first RFcoil and at least one additional RF coil, the first RF coil providing animage signal; passing the image signal from the first RF coil through afirst transformer to obtain a transformed signal; matching impedance ofthe first transformer to an input impedance of a preamplifier; andamplifying the transformed signal with the preamplifier.

One embodiment of the invention includes an apparatus for imaging,including: a RF coil array including a plurality of RF coils, the RFcoil array adapted to generate an image signal, the RF coils beinginductively decoupled from each other; and a plurality of preamplifiers,each preamplifier adapted to receive the image signal from a respectiveRF coil, a ground for each preamplifier being electrically isolated froma ground for each respective RF coil.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features of various embodiments of the inventionwill be apparent from the following, more particular description of suchembodiments of the invention, as illustrated in the accompanyingdrawings, wherein like reference numbers generally indicate identical,functionally similar, and/or structurally similar elements. Theleft-most digit in the corresponding reference number indicates thedrawing in which an element first appears.

FIG. 1 illustrates a circuit diagram according to an exemplaryembodiment of the invention.

FIG. 2A illustrates a circuit diagram according to an exemplaryembodiment of the invention.

FIG. 2B illustrates a circuit diagram according to an exemplaryembodiment of the invention.

FIG. 3 illustrates a circuit diagram according to an exemplaryembodiment of the invention.

FIG. 4 illustrates a circuit diagram according to an exemplaryembodiment of the invention.

FIG. 5A shows individual fast low-angle shot (FLASH) images of coils inan exemplary four coil array.

FIG. 5B shows the sum of squares reconstruction for the images of FIG.5A.

FIGS. 6A, 6B, and 6C show images acquired with generalizedautocalibrating partially parallel acquisition (GRAPPA) reconstructionat different acceleration factors for the exemplary four coil array.

DETAILED DESCRIPTION

Exemplary embodiments of the invention are discussed in detail below.While specific exemplary embodiments are discussed, it should beunderstood that this is done for illustration purposes only. Indescribing and illustrating the exemplary embodiments, specificterminology is employed for the sake of clarity. However, the inventionis not intended to be limited to the specific terminology so selected. Aperson skilled in the relevant art will recognize that other componentsand configurations may be used without parting from the spirit and scopeof the invention. It is to be understood that each specific elementincludes all technical equivalents that operate in a similar manner toaccomplish a similar purpose. Each reference cited herein isincorporated by reference. The examples and embodiments described hereinare non-limiting examples.

According to the exemplary embodiments of the invention, the RF coils ina RF coil array of a MRI system are inductively decoupled from eachother. Each RF coil is coupled to its respective preamplifier with atleast one transformer. A nearly perfect balanced signal is transferredto the preamplifier from the RF coil, and at the same time, the RF coilbecomes electrically isolated from the preamplifier and its associatedelectronics. The electrical isolation may reduce some of the undesiredeffects of ground loops and parasite signals that typically appear incapacitively-coupled networks. The relationship between the turn ratiofor the transformer and the input impedance of the preamplifierdetermines the decoupling intensity between the RF coils and also thesignal-to-noise ratio for the amplified signal output from thepreamplifier.

With the invention, the RF coils may be positioned according todifferent geometrical configurations, such as in a parallel arrangement,an overlapped mode, separated by gaps, or even stacked on top of eachother.

With the invention, each preamplifier is connected to a separate receivechannel of the MRI system, and the channels may be accessedindependently and simultaneously, making it possible to perform parallelimaging acquisitions.

The MRI system may be also used for magnetic resonance spectroscopy(MRS). Other imaging embodiments may further be used with the invention.

FIG. 1 illustrates a circuit diagram according to an exemplaryembodiment of the invention. The MRI imaging system includes, forexample, an RF coil array having at least two RF coils L1 and L2. Thecoil array 8 may include additional coils (not shown). Each coil in thecoil array is coupled to its own respective preamplifier. Coil L1 iscoupled to preamplifier P1, and coil L2 is coupled to preamplifier P2.The two coils L1 and L2 may have different inductances and may have amutual coupling M.

The network circuit 10 for coil L1 includes a capacitor C12 connected inparallel to a transformer T1. This parallel circuit is connected inseries through a capacitor C11 to coil L1. The transformer T1 couplescoil L1 to its respective preamplifier P1. The transformer T1 is coupledto coil L1 through its primary coil 12 and to the preamplifier P1through its secondary coil 13. The secondary coil 13 of transformer T1is connected in series through a capacitor C13 to the preamplifier P1,which has an input impedance of Zp1. The primary and secondary coils 12,13 of transformer T1 may have a turn ratio of n:1, where n is greaterthan 0, including fractional numbers. Through the capacitor C13, theimpedance of the secondary coil 13 of transformer T1 matches the inputimpedance Zp1 of the preamplifier P1. The transformer T1 has a couplingcoefficient K1. The network circuit 11 for coil L2 is similar to thenetwork circuit 10 for coil L1 and couples coil 12 to the preamplifierP2 via the transformer T2. The impedance of the secondary coil 13 of thetransformer T2 matches the impedance of the preamplifier P2. The groundGND3 of circuit 10 is electrically isolated from the ground GND2 ofcircuit 11 and from the ground GND1 of the preamplifier P1. Thepreamplifiers P1 and P2 are connected to the ground GND1.

The turn ratio of transformers T1 and T2 may be selected to minimizenoise for the preamplifiers P1 and P2, respectively. A number of windingin the primary coil of the transformer may be equal to, greater than, orfewer than a number of windings in the ns secondary coil of thetransformer. Further, the isolation limits for coils L1 and L2 in coilarray 8 may be enhanced by using high input impedance for thepreamplifiers P1 and P2, respectively.

The transformers T1 and T2 are passive transformers. The transformer T1,T2 may include helical windings, solenoidal windings, or striplinebaluns. The transformer T1, T2 may include an electrostatic shieldcomprised of a conducting material. The transformer T1, T2 may utilizesuperconducting materials.

The signal that reaches the preamplifier P1 is coupled inductively tocoil L1 through the primary coil of the transformer T1. Because theprimary and secondary coils of the transformer T1 are isolated, thepreamplifier circuit (and the MRI scanner electronics connected thereto(not shown)) are electrically isolated from coil L1. This arrangementprovides an electrical balance and isolation between the channels of thecoil array. With the Invention, traps and baluns in the circuits 10 and11, a used conventionally, may be unnecessary. Further, the inductivelycoupled inventive technique employs a transformer to match each coil inthe coil array to its respective preamplifier. The inventive techniqueprovides a balanced structure that may reduce dielectric losses and mayisolate the lossy currents in ground loops.

Referring to the exemplary circuit diagram of FIG. 1, it may be shownthat the impedance of the circuit 10 as seen from the circuit 11, may begiven as:

$\begin{matrix}{Z = {R_{2} + {\frac{\omega^{2} \cdot M^{2} \cdot L^{2}}{R_{1} + \frac{n^{2} \cdot Z_{p\; 1}}{K_{1}^{2}}}.}}} & (1)\end{matrix}$

where ω is the NMR Larmor frequency, M is the inductive coupling betweenL1 and L2, L is the inductance of coil L2, R1 is the resistance of coilL1, R2 is the resistance of coil L2, K1 is the coupling coefficient forthe transformer T1, n is from the turn ratio n:1 between the primary andsecondary coils 12, 13 of the transformer T1, and Zp1 is the inputimpedance of the preamplifier P1. In order to achieve high isolationbetween coils L1 and L2, Z should be made as close as possible to R2.Thus, the second term in equation (1) should be made as negligible aspossible. This can be accomplished if both n and Zp1 are chosen to behigh.

The exemplary embodiment, described above with reference to FIG. 1, wastested in a 7T/30 cm Bruker Avance MRI system. A four-coil array forimaging a rat brain was built using the circuit described with referenceto FIG. 1. A positive-intrinsic-negative (PIN) diode circuitry (notshown in the circuit of FIG. 1) was incorporated to allow decoupling ofthe coil array from the transmit coil. The coil array was connected toregular 50 Ω input impedance preamplifiers. The transformers were madevery small with a 7:1 turn ratio between the primary coil and thesecondary coil. No trap or baluns were employed in the circuit array.The isolation level for this configuration is described by equation (1),and this is equivalent to the isolation achieved when using low inputimpedance preamplifiers, with, for example, 1Ω input impedance.

The measured isolation between the channels in the test was better than45 dB. FIG. 5A shows individual FLASH axial images obtained with eachcoil, and FIG. 5B shows a combined image obtained using a sum of squaresreconstruction from the four images shown in FIG. 5A. As can be seen bycomparing the individual images in FIG. 5A to the combined image in FIG.5B, no significant coupling is observed between the RF coils. FIGS. 6A,6B, and 6C show FLASH images acquired with the GRAPPA acquisition schemeat three different acceleration factors (AF), 1, 2, and 2.91,respectively. Reconstruction artifacts are only noticeable for AF=2.91in FIG. 6C in the form of aliasing banding along the phase encodingdirections. The inductive decoupling between the different channelsshows good performance with excellent isolation of all four channels andimmunity to standing waves or other parasitic signals.

FIG. 2A illustrates a circuit diagram according to an exemplaryembodiment of the invention. The circuit diagram of FIG. 2A depictsanother technique to inductively decouple the coils L1 and L2, asdiscussed above for the exemplary embodiment of FIG. 1. The circuitdiagram in FIG. 2A depicts network circuit 20, for example, a baluncircuit, for coupling the coil L1 to the preamplifier P1 via thetransformer T1 and depicts network circuit 21, for example, a baluncircuit, for coupling the coil L2 to the preamplifier P2 via thetransformer T2. The balun circuit 20 for coil L1 includes a capacitorC20, capacitors Ca and Cb, inductors La and Lb, a capacitor Cm21, andthe transformer T1. The balun circuit 20 converts the low impedance ofthe transformer T1 into a high impedance for the coil L1. The capacitorCm22 matches impedance of the transformer T1 to the input impedance Zp1of the preamplifier P1 via the secondary coil 22 of transformer T1. Thecircuit 21 for coil L2 is similar to the circuit 20 for coil L1 andmatches impedance of the coil L2 to the impedance of the preamplifierP2. The ground GND3 of circuit 20 is electrically isolated from theground GND2 of circuit 21 and from the ground GND1 of the preamplifierP1.

Referring to an exemplary circuit diagram of FIG. 2A, it may be shownthat the impedance of circuit 20 as seen from the resistive impedance ofL1 is given by:

$\begin{matrix}{Z = {n^{2} \cdot \frac{1}{Z_{p\; 1}} \cdot \frac{L}{C}}} & (2)\end{matrix}$

where n is the turn ratio between the secondary and the primary coils ofthe transformer T1, Zp1 is the impedance of the preamplifier P1, L isthe value for the inductances La or Lb (La=Lb=L), C is the value for thecapacitor Ca or Cb (Ca=Cb=C). Equation (2) assumes that the primary andsecondary resistances of the transformer T1 are negligible and also thatthe coupling coefficient K1 for the transformer T1 is 1. For bestdecoupling of the coils L1 and L2, the impedance load in circuit 20 asseen from coil L1 should be high, i.e., Zp1 should be made as large aspossible. According to equation (2), the presence of the transformer T1in circuit 20 enhances the impedance seen from coil L1 by a factor ofn².

With reference to FIG. 2B, this exemplary embodiment is similar to anexemplary embodiment of FIG. 2A except that each balun circuit 20, 21 inFIG. 2A is replaced by a coaxial cable 24, 25 with a lengthsubstantially equal to one fourth of a wavelength at the frequency ofoperation. The coaxial cable 24 converts the low impedance of thetransformer T1 into a high impedance for the coil L1. Of course, it iscontemplated that the coaxial cable 24, 25 may be replaced with anequivalent circuit representing an equivalent of one fourth of awavelength at the frequency of operation.

The capacitor Cm22 matches impedance of the transformer T1 to the inputimpedance Zp1 of the preamplifier P1 via the secondary coil 22 oftransformer T1. The coaxial cable is connected at each end in serieswith the capacitors Cm20, Cm21. The capacitors Cm20, Cm21 may be lengthcompensation capacitors that are configured to cancel at least some ofthe phase shift in the coaxial cable. The values of the capacitors Cm20,Cm21 may be selected based upon the length of the coaxial cable and thedesired operating characteristics. The coaxial cable has shown bettereffective signal transmission than the balun circuit of an exemplaryembodiment of FIG. 2A.

FIG. 3 illustrates a circuit diagram according to an exemplaryembodiment of the invention. The circuit diagram of FIG. 3 depictsanother technique to inductively decouple the coils L1 and L2, asdiscussed above for the exemplary embodiment of FIG. 1. The circuitdiagram in FIG. 3 depicts network circuit 30, which differs from networkcircuit 10 in FIG. 1, for coupling the coil L1 to the preamplifier P1via the transformers T1, T3, and T4 and depicts network circuit 31,which differs from network circuit 11 in FIG. 1, for coupling the coilL2 to the preamplifier P2 via the transformers T2, T5, and T6. Incircuit 30, the detected signal in the coil L1 is distributed in abalanced configuration to transformers T3 and T4 at the same time. Theoutputs from the transformers T3 and T4 are amplified in preamplifiersP3 and P4, respectively, and combined in transformer T1. Impedance ofthe transformer T1 is matched to impedance of the MRI receiver chain,e.g. the input impedance of the preamplifier P1. Since noise signalscoming from preamplifiers P3 and P4 are not correlated, the combinationof signals may provide better signal-to-noise ratio as compared to thecircuit that uses just one preamplifier as shown in FIG. 1. The circuit31 for coil L2 is similar to the circuit 30 for coil L1. The ground GND3of circuit 30 is electrically isolated from the ground GND2 of circuit31 and from the ground GND1 of the preamplifier P1. The uncouplingbetween coils L1 and L2 for this exemplary embodiment is twice aseffective as the uncoupling between the coils L1 and L2 for theexemplary embodiment of FIG. 1.

FIG. 4 illustrates a circuit diagram according to an exemplaryembodiment of the invention. The circuit diagram of FIG. 4 depictsanother technique to inductively decouple the coils L1 and L2, asdiscussed above for the exemplary embodiment of FIG. 1. The circuitdiagram in FIG. 4 depicts network circuit 40, which differs from networkcircuit 10 in FIG. 1, for coupling the coil L1 to the preamplifier P1via the transformer T1 and depicts network circuit 41, which differsfrom network circuit 11 in FIG. 1, for coupling the coil L2 to thepreamplifier P2 via the transformer 12. The circuit 40 includescapacitors C41, C42 and C43, an inductor L43, and the transformer T1.Through the capacitor Cm41, the impedance of the secondary coil 42 oftransformer T1 matches the input impedance Zp1 of the preamplifier P1.The circuit 41 for coil L2 is similar to the circuit 40 for coil L1. Theground GND3 of circuit 40 is electrically isolated from the ground GND2of circuit 41 and from the ground GND1 of the preamplifier P1.

To achieve high levels of decoupling between the coils L1 and L2, thecircuits 40 and 41 require low input impedance for the preamplifiers P1and P2, respectively. The transformers T1 and T2 can be used to lowerthe equivalent input impedance of the preamplifiers P1 and P2,respectively. For circuit 40 (and similarly for circuit 41), it may beshown that the equivalent input impedance Zin of the preamplifier P1measured on the primary coil 43 of transformer T1 is given by:

$\begin{matrix}{Z_{in} = \frac{Z_{p\; 1}}{n^{2}}} & (3)\end{matrix}$

where Zp1 is the input impedance of the preamplifier P1, and n is theturn ratio between the secondary and the primary coils 42, 43 of thetransformer T1. According to equation (3), the transformer T1 lowers theequivalent input impedance Zin of the preamplifier P1 by a factor of n².

The invention is described in detail with respect to exemplaryembodiments, and it will now be apparent from the foregoing to thoseskilled in the art that changes and modifications may be made withoutdeparting from the invention in its broader aspects, and the invention,therefore, as defined in the claims is intended to cover all suchchanges and modifications as fall within the true spirit of theinvention.

1. An apparatus for imaging, comprising: a radio frequency (RF) coilarray comprising a first RF coil and at least one additional RF coil,the RF coil array adapted to generate an image signal; a preamplifierhaving an input impedance, the preamplifier adapted to receive the imagesignal from the first RF coil; and a first transformer to couple thefirst RF coil to the preamplifier, impedance of the first transformer tomatch the input impedance of the preamplifier.
 2. The apparatus of claim1, wherein the first transformer comprises a primary coil coupled to thefirst RF coil and a secondary coil coupled to the preamplifier.
 3. Theapparatus of claim 2, wherein a number of windings for the primary coilof the transformer is one of greater than, fewer than, or equal to anumber of windings for the secondary coil of the transformer.
 4. Theapparatus of claim 1, further comprising: a second transformer; and athird transformer, wherein the first transformer, the secondtransformer, and the third transformer couple the first RF coil to thepreamplifier.
 5. The apparatus of claim 1, wherein the first transformercomprises at least one of helical windings, solenoidal windings, orstripline baluns.
 6. The apparatus of claim 1, wherein the firsttransformer is a passive transformer.
 7. The apparatus of claim 1,wherein the first transformer comprises an electrostatic shieldcomprising a conducting material.
 8. The apparatus of claim 1, whereinthe first transformer comprises a superconducting material.
 9. Theapparatus of claim 1, wherein a ground of the first RF coil iselectrically isolated from a ground of the preamplifier.
 10. Theapparatus of claim 1, wherein a ground of the first RF coil iselectrically isolated from grounds of the additional RF coils in the RFcoil array.
 11. The apparatus of claim 1, wherein the apparatus isadapted for magnetic resonance imaging or magnetic resonancespectroscopy.
 12. The apparatus of claim 1, further comprising: a baluncircuit to couple the first RF coil to the first transformer.
 13. Theapparatus of claim 1, further comprising: a coaxial cable with a lengthequal to one fourth of a wavelength at a frequency of an operation ofthe first RF coil, to couple the first RF coil to the first transformer.14. A method for imaging, comprising: obtaining an image with a radiofrequency (RF) coil array comprising a first RF coil and at least oneadditional RF coil, the first RF coil providing an image signal; passingthe image signal from the first RF coil through a first transformer toobtain a transformed signal; matching impedance of the first transformerto an input impedance of a preamplifier; and amplifying the transformedsignal with the preamplifier.
 15. The method of claim 14, wherein thefirst transformer comprises primary and secondary coils and furthercomprising: coupling the primary coil to the first RF coil; and couplingthe secondary coil to the preamplifier.
 16. The method of claim 14,further comprising: coupling the first RF coil to the first transformervia second and third transformers; and coupling the first coil to thefirst transformer with the second and third transformers.
 17. The methodof claim 14, further comprising: coupling the first RF coil to the firsttransformer via a balun circuit.
 18. The method of claim 14, furthercomprising: coupling the first RF coil to the first transformer via acoaxial cable with a length equal to one fourth of a wavelength at afrequency of an operation of the first RF coil.
 19. An apparatus forimaging, comprising: a radio frequency (RF) coil array comprising aplurality of RF coils, the RF coil array adapted to generate an imagesignal, the RF coils being inductively decoupled from each other; and aplurality of preamplifiers, each preamplifier adapted to receive theimage signal from a respective RF coil, a ground for each preamplifierbeing electrically isolated from a ground for each respective RF coil.20. The apparatus of claim 19, further comprising: a plurality oftransformers, each transformer to couple one of the RF coils to arespective preamplifier so that impedance of each transformer matches aninput impedance of the respective preamplifier.